OPR1000 중대사고 완화전략의 효용성 및 부작용에 대한 전산적 평가

Abstract

후쿠시마 사고가 발생한 이후로, 중대사고에 대한 많은 연구들이 수행되고 있다. 특히, 중대사고 지침서를 포함한 중대사고 안전규제법이 국회 본회의를 통과하였다. 그러나 많은 시간과 비용의 한계로 인하여 중대사고 완화전략에 대한 연구가 부족한 현실이다. 따라서 본 연구는 한국표준형원전을 대상으로 독립적인 완화전략을 적용하여 전산해석을 수행하였고, 각각의 유효성과 부작용을 확인하였다. 전산해석은 중대사고 해석 코드인 MELCOR 1.8.6을 이용하여 중대사고 평가방법론에 따라 수행하였다. 수행 내용은 다음과 같다. 첫 번째, 한국표준형원전을 참조원전으로 선정하였다. 두 번째, 안전주입이 없는 소형파단 냉각재상실사고, 전원상실사고, 완전급수상실사고를 중대사고 초기사건으로 가정하였다. 세 번째, 사고 상황에서 가용할 수 있는 완화전략의 조치사항을 선정하였다. 네 번째, 선정된 완화전략의 조치사항을 적용하였을 경우, 유효성과 부작용을 확인하였다. 마지막으로, 완화전략의 효용성을 네 번째 단계에서 수행한 유효성과 부작용으로 평가하였다. 전산해석 결과 완화전략-01 증기발생기 냉각수 주입이 가장 효과적인 완화전략으로 평가되었다. 완화전략-02 원자로냉각재계통 감압은 1차 계통의 증기가 격납건물로 방출되어 핵분열생성물의 방출을 촉진시키는 부작용을 야기함을 확인하였다. 완화전략-07의 조치사항인 피동형수소재결합기를 통하여 수소 폭발의 가능성을 줄일 수 있었다. 그러나 재결합 과정에서 발생한 증기와 발열로 인하여 격납건물의 압력이 증가하여 건전성에 위해가 될 수 있음을 확인하였다. 그리고 완화전략-04,05,06의 조치사항인 살수계통을 적용하면 격납건물의 압력이 감소하였지만, 격납건물 내 수소 농도가 증가하여 수소 폭발의 가능성이 증가하여 격납건물의 건전성을 위협하였다. 결론적으로, 사고에 따라 완화전략을 적용하였을 경우, 발생할 수 있는 부작용을 확인하고 완화전략의 조치사항을 신중히 선택해야 함을 도출하였다. | After severe accident happened in Fukushima Daiichi nuclear power plants (NPPs), many studies for severe accident have been performed. Especially, nuclear safety laws which enforces development of Severe Accident Management Guidance (SAMG) is allowed in the assembly plenary session. However, there is a lack of research on mitigation strategies for severe accident due to the considerable time and expenditures. The objective of research is to evaluate effectiveness and adverse effects on individual mitigation strategies of SAMG for OPR1000. The numerical evaluation was performed using MELCOR 1.8.6 code according to the evaluation methodology for severe accident mitigation strategies. First, OPR1000 was selected for the reference NPP. Second, Small Break Loss of Coolant Accident without Safety Injection, Station Black Out, and Total Loss of Feed Water were selected as the postulated severe accident scenarios. Third, the feasible mitigation measures were chosen for the mitigation strategies. Fourth, the effectiveness and adverse effects on individual severe accident mitigation measure were analyzed in terms of reactor pressure vessel failure, oxidation heat generation, hydrogen concentration, and delay of fission products (FPs) leak into the environment. Fifth, the benefit of severe accident mitigation measure were evaluated using the effectiveness and adverse effects. The numerical evaluation shows that the first mitigation strategy, Mitigation 1, feeding water into the steam generators is the most effective among the other mitigations. In addition, Mitigation 2, reactor coolant system depressurization alone could not mitigate the severe accident sufficiently. Among four ex-vessel mitigation strategies, Mitigation 7, a reliable operation of Passive Autocatalytic Recombiners (PARs) was essential in reducing the hydrogen risk. Nonetheless, local heating and steam generation due to catalytic reaction could be an issue threating the containment integrity. It was also shown that use of spray to control the FPs and containment state may result in undesirable adverse effect of condensing steam and subsequent increase of partial pressure of hydrogen, which may contribute on raising the possibility of hydrogen combustion. In conclusion, this study suggests that mitigation measures should be carefully selected and encountering measures should be prepared for the possible adverse effects.; After severe accident happened in Fukushima Daiichi nuclear power plants (NPPs), many studies for severe accident have been performed. Especially, nuclear safety laws which enforces development of Severe Accident Management Guidance (SAMG) is allowed in the assembly plenary session. However, there is a lack of research on mitigation strategies for severe accident due to the considerable time and expenditures. The objective of research is to evaluate effectiveness and adverse effects on individual mitigation strategies of SAMG for OPR1000. The numerical evaluation was performed using MELCOR 1.8.6 code according to the evaluation methodology for severe accident mitigation strategies. First, OPR1000 was selected for the reference NPP. Second, Small Break Loss of Coolant Accident without Safety Injection, Station Black Out, and Total Loss of Feed Water were selected as the postulated severe accident scenarios. Third, the feasible mitigation measures were chosen for the mitigation strategies. Fourth, the effectiveness and adverse effects on individual severe accident mitigation measure were analyzed in terms of reactor pressure vessel failure, oxidation heat generation, hydrogen concentration, and delay of fission products (FPs) leak into the environment. Fifth, the benefit of severe accident mitigation measure were evaluated using the effectiveness and adverse effects. The numerical evaluation shows that the first mitigation strategy, Mitigation 1, feeding water into the steam generators is the most effective among the other mitigations. In addition, Mitigation 2, reactor coolant system depressurization alone could not mitigate the severe accident sufficiently. Among four ex-vessel mitigation strategies, Mitigation 7, a reliable operation of Passive Autocatalytic Recombiners (PARs) was essential in reducing the hydrogen risk. Nonetheless, local heating and steam generation due to catalytic reaction could be an issue threating the containment integrity. It was also shown that use of spray to control the FPs and containment state may result in undesirable adverse effect of condensing steam and subsequent increase of partial pressure of hydrogen, which may contribute on raising the possibility of hydrogen combustion. In conclusion, this study suggests that mitigation measures should be carefully selected and encountering measures should be prepared for the possible adverse effects.